The RDR-4000 IntuVue® radar incorporates a volumetric, 3D buffer concept that enables several features not previously available in airborne weather radar. One of those features is the Automatic mode which provides a display of weather relevant to the intended aircraft flight path (either via data from a flight management system or inferred from present altitude, vertical speed and/or ground speed).
Currently, the relevant weather is defined as a corridor (envelope) extending from 4000 ft above the intended aircraft flight path to 4000 ft below the aircraft intended flight path, with the following basic modifications:                the lower boundary is limited to a maximum value of 25,000 ft MSL;        the lower boundary is limited to a minimum value of 0 ft MSL (limit of the volumetric buffer);        the upper boundary is limited to a maximum value of 60,000 ft MSL (limit of the volumetric buffer); and        the upper boundary is limited to a minimum of 10,000 ft AGL or MSL (depending on aircraft type).        
FIG. 1 illustrates these modifications.
The weather or more specifically, the reflectivity and turbulence data that is within the envelope is displayed on a horizontal display in solid colors (green, yellow, red). Reflectivity is the characteristic of weather that the radar system measures. Reflectivity provides a rough estimate of rainfall rate. The turbulence indications (usually display in magenta) indicate areas estimated by the radar to contain severe turbulence in areas of radar-detectable precipitation.
The color displayed at any point on the display is determined by the maximum reflectivity in the portion of the column above the earth inside the relevant envelope. For example, if any portion of the column inside the relevant envelop is red, the displayed value at the corresponding display point will be red. If the maximum value is yellow, the displayed value at the corresponding display point will be yellow, and similarly for green.
Weather that is outside the envelope is referred to as “non-relevant” weather and can either be suppressed (not displayed) or displayed as cross-hatched colors (depending on crew selection). If enabled for display, it is only displayed in areas where the relevant weather is below the green threshold. For display priority, any green or above relevant weather is overlaid on all non-relevant weather, for example, green relevant has priority over red non-relevant. An example of a display with the relevant (solid colors) and non-relevant (cross-hatched colors) is shown in FIG. 2.
The 25,000 ft limit on the lower boundary of the envelope was selected for meteorological reasons associated with the characteristics of convective weather (e.g. thunderstorms). Inherently, convective weather has vertical winds which can represent hazards to aircraft. The extent of these potentially hazardous vertical winds is not necessarily confined to or associated with regions of high reflectivity.
One of the reasons for this is that above the freezing altitude (the altitude of the 0° C. isotherm), much of the water is frozen. The higher the altitude above the freezing altitude, the higher percentage of frozen water. Frozen water (“ice”) does not return as strong a signal to the radar as liquid water. All other things being equal (particle size and density), ice returns a factor of about 5 times less power (7 dB) than liquid water.
Although the radar compensates for the lower reflectivity factor of frozen water, to ensure that the most reflective part of a thunder cell is presented to the pilot, it is necessary that the part of the convective cell at or below the freezing altitude is included in the relevant envelope. Hence the selection of 25,000 ft which represents a good average 0° C. isotherm under conditions at which hazardous convective activity can exist, or at least represents a safe altitude at which high reflectivity will occur.
Operational experience over the last few years (since circa 2005) has indicated that under some circumstances, the use of the 25,000 ft Maximum Lower Boundary (MLB) has some side effects. These circumstances include high altitude operation (above 35,000 ft) in the presence of stratiform type weather or low-level and non-threatening convection.
Stratiform (non-convective) weather is characterized by low or non-existent vertical winds and is generally safe to fly above or through. It does not generally extend vertically to the altitudes that convective weather may, but it can on occasion extend to 25,000 ft. Non-threatening convective weather can also top out in the 25,000 ft range.
Further the altitude at which the radar “perceives” reflectivity to exist can be expanded to some degree by the fact that the beamwidth of the radar beam cannot be indefinitely made narrow. The degree to which the antenna beam “expands” the perceived reflectivity is a function of the beamwidth and the range from the radar to the weather. The antenna beamwidth narrowness is limited by physics for a given operating frequency and antenna size in an inverse relationship; that is beamwidth decreases with higher frequency and larger physical antenna size.
Therefore, there are occasions in which aircraft are flying at high altitudes where non-threatening weather below is indicated as “relevant”, due to the 25,000 ft MLB. Further, these relevant indications often turn out to be non-relevant as the aircraft approaches 40 nm and the vertical beam extent decreases. This is later than is desirable for making an operational decision to efficiently avoid weather (i.e., if it is necessary to fly around or climb above weather, it is better to do it sooner rather than later).